Fermi and The Search for ET
Welcome back! To recap, much has been said about the existence of extraterrestrial intelligence (ETI) over the years. Similarly, a lot has been said about why we haven’t found any evidence of it and why. This is the essence of the “Fermi Paradox,” which boils down to all of our attempts to answer why we haven’t heard from any aliens (yet!)
This raises an important question that came up at the tail end of the last installment (Fermi and the “Great Filter”). How exactly are we looking for extraterrestrial life? Equally important, could this have any bearing on the fact that we haven’t found hard evidence of any yet? I say “hard” to differentiate it from anecdotal and unresolved instances (more on that later).
To break it down, the Search for Extraterrestrial Intelligence (SETI) comes down to two categories — Biosignatures and Technosignatures. Biosignatures amount of indicators that we associate with life or biological processes — in short, signs of life. Technosignatures, on the other hand, are signatures we associate with technological activity. So if biosignatures are an indication of life, technosignatures are indications of advanced life.
With the broad definitions covered, let’s talk specifics…
Biosignatures
Biosignatures (or biomarkers) can include the presence of atmospheric chemicals like oxygen gas, carbon dioxide, nitrogen gas, methane, water, and others. These are indicative of life because they are either a) essential to its functions, b) by-products of its biological functions, or c) associated with habitable environments as we know them (in other words, Earth).
Earth’s atmosphere is composed of 78% nitrogen gas and 21% oxygen gas, with trace amounts of other gases (like 0.4% carbon dioxide) and water vapor. Nitrogen gas is an important buffer in our atmosphere and is converted (through the nitrogen cycle) into chemicals like ammonia, nitrates, and nitrites, some of which are essential to soil fertility.
Oxygen gas is absolutely essential for complex life forms (including us!), which metabolize it and produce carbon dioxide as a by-product. Meanwhile, photosynthetic organisms — ranging from plants to prokaryotes (like cyanobacteria) — metabolize carbon dioxide and produce oxygen gas as a by-product. This cycle is linked to the evolution of Earths’ atmosphere, which was predominantly composed of carbon dioxide billions of years ago.
Carbon dioxide has also played an important role in maintaining the stability of our climate. Over time, Earth has gone through periods of glaciation (ice ages) and melting due to changes in its axial tilt (obliquity) and changes in its orbit. The presence of sufficient amounts of carbon dioxide (and other greenhouse gases) has ensured that our planet has had enough of a greenhouse effect to maintain stable temperatures.
Then there’s methane, which is generally created from the decay of organic matter, is also a by-product of certain mammal’s digestive process (like cows), and (as well as ammonia) is a super-greenhouse gas that has maintained stability in Earth’s climate. All of these gases essential to life and have played an essential role in the evolution of terrestrial lifeforms.
Another interesting biosignature comes in the form of pigment and the wavelengths at which light is absorbed and radiated from a planet. This method was endorsed by famed astronomer and science communicator Carl Sagan, who claimed that scientists could discern the presence of photosynthesis this way.
Basically, green plants absorb red and yellow light and reflect green light (hence their color) while also glowing brightly in the infrared wavelengths. For decades, Earth observation satellites have relied on this phenomenon — called the Vegetation Red Edge (VRE) — to monitor forests and vegetation from orbit. With the right sensitivity and resolution, telescopes could look for VRE signatures on exoplanets.
There’s also retinal-based photosynthesis, where light is most strongly absorbed in the green-yellow parts of the spectrum, and purple light is reflected. This process may predate chlorophyll-based photosynthesis on Earth by hundreds of millions of years. By looking for purple light, astronomers could look for signs of very early lifeforms.
Technosignatures
The difference between biosignatures and technosignatures is what separates the search for extraterrestrial life (astrobiology) and the Search for Extraterrestrial Intelligence (SETI). Whereas astrobiology is concerned with the study of life in the cosmos under any and all regimens, SETI is about finding life “like us” — i.e., technologically-dependent beings we might be able to communicate with.
By definition, a technosignature is any indication of possible technological activity. In this respect, scientists engaged in SETI are on the lookout for technologies that we know work — either through experience or because they rely on known physics. The term was coined by Jill Tarter, who has been the director of the Center for SETI Research (SETI Institute) for 35 years and was a project scientist for NASA’s SETI program before it was canceled in 1993.
In an article she wrote in 2007, titled “The evolution of life in the Universe: Are we alone?” she said the following about the search for technosignatures:
“If we can find technosignatures — evidence of some technology that modifies its environment in ways that are detectable — then we will be permitted to infer the existence, at least at some time, of intelligent technologists. As with biosignatures, it is not possible to enumerate all the potential technosignatures of technology-as-we-don’t-yet-know-it, but we can define systematic search strategies for equivalents of some 21st-century terrestrial technologies.”
The subject of technosignatures was discussed extensively at the 2018 NASA Technosignatures Workshop, which was accompanied by a report titled “NASA and the Search for Technosignatures.” Of all the technosignatures ever considered and investigated, radio communication signals are the most time-honored. While the report concedes that no artificial signals have been confirmed to date, it is still viable as far as SETI research is concerned.
However, many other methods have been proposed based on technologies that are known to be feasible (at least in theory). These include optical transmissions (lasers), microwave transmissions, quantum transmissions, Fast Radio Bursts (FRBs), neutrino beams, and even gravitational waves. Beyond communications, a number of other activities (and how we might look for them) have been raised in the past half-century.
These include Freeman Dyson’s proposal that a sufficiently advanced species would be capable of engineering structures so massive, they would encompass an entire solar system. In Dyson’s case, he envisioned a massive sphere (a Dyson Sphere) that would draw power from the parent star 24/7 and provide enough room for trillions of inhabitants.
Variations on this theme include star-encircling rings (aka. “Ringworlds,” or Niven Rings), swarms of tiny structures, clouds of computronium, overlapping shells, massive “islands” in space, and others. These are often referred to collectively as “Dyson Structures.” Similar suggestions have been made about smaller “megastructures” or other examples of advanced engineering.
They include large belts of artificial satellites around a planet (aka. Clarke Belts), signs of nuclear testing in space, radiation caused by nuclear propulsion, artificial lighting from urban centers, and concentrations of synthetic chemicals and carbon dioxide that would indicate industrial pollution. Each of these signatures (bio and techno) present their own share of advantages and problems, but we’ll save that for later.
Detection Methods
Right now, the biggest challenge is detecting these signatures, which is a matter of instrumentation and methodology. In rare cases, astronomers are able to obtain spectra from the atmospheres of exoplanets using Transit Photometry (aka. the Transit Method). This entails monitoring stars for periodic dips in brightness, which could be an indication of a planet passing in front of the star (aka. “transiting”) relative to the observer.
At times, the light passing through the transiting exoplanet’s atmosphere can be studied via spectroscopy to determine what chemical compounds are present. However, the most effective method for determining atmospheric composition is Direct Imaging, where scientists observe light reflected directly from an exoplanet’s surface and/or atmosphere. In these cases, astronomers can tell which wavelengths of light are absorbed and which are radiated.
This is not only an effective means for detecting the chemical compounds associated with life but also for detecting evidence of photosynthesis (by searching for the VRE). Unfortunately, the only exoplanets that have been detected to date using this method have been rather massive (gas giants) and/or had distant orbits with their parent stars.
Luckily, next-generation telescopes are coming online in the next few years that will change all that. These include the James Webb Space Telescope (JWST), the Nancy Grace Roman Space Telescope (RST), the Extremely Large Telescope (ELT), and Giant Magellan Telescope (GMT) in Chile, and the Thirty Meter Telescope (TMT) in Hawaii.
With their advanced infrared-imaging suites, chronographs, spectrometers, and adaptive optics, these observatories will be able to conduct direct imaging studies of planets that are smaller and orbit closer to their stars. This is where smaller, rocky planets that orbit within their parent star’s Habitable Zone (HZ) — aka. “Earth-like” planets — are most likely to reside.
Since astrobiologists generally consider these planets to be the best place to find extraterrestrial life, these instruments are expected to help narrow the search immensely. Of course, there’s also the age-old radio astronomy methods, where astronomers will aim their telescopes at what they think will be a promising target and try to “tease out” signals from the noise.
And of course, there are surveys that look for visible and non-visible electromagnetic radiation, which includes spillover from directed-energy (DE) communications (or propulsion), gamma-rays, X-rays, ultraviolet rays, or microwaves, any of which could be indications of transmission technology.
The “Low-Hanging Fruit” Approach
Alas, there’s an important question which (in my experience) is asked whenever the subject of SETI and exoplanet research comes up: “Why are we looking for life as we know it. Shouldn’t we be looking for life as we don’t know it?” As questions go, this one is right up there with, “What’s the point of space exploration? Shouldn’t we fix Earth first?”
It’s a frustrating question, partly because it’s understandable and partly because it’s misinformed, but also because it takes time to unpack. Luckily for me, my boss and publisher (Fraser Cain, Universe Today) once answered this question in a way that I found very succinct and inspired. He even put it in a video (shown below), so he wouldn’t have to explain it over and over. Basically, he said, we are following the “low-hanging fruit” approach.
To put it simply, we know of only one planet where life exists (Earth) and one intelligent species that is heavily reliant on technology (us). So if there is life out there that emerged under conditions and via evolutionary pathways that are unfamiliar to us, how would we even know what to look for? Sure, there are theoretical studies on the matter that have offered some interesting possibilities (such as methanogenic life on Titan).
But until we find it and study it, we won’t know how to spot it over vast distances (like hundred or thousands of light-years). Short answer: we can’t look for life as we don’t know it because we don’t know it! If we want to find exotic life out there, we need to know what to look for. But we won’t know much about that until we find some. It’s a dilemma, to be sure, but it manages to capture some of the harsh realities of SETI.
In terms of scientific inquiry, it is the most data-poor one there is. To make matters worse, we don’t know nearly as much about life “as we know it” as you might think. How did life emerge here on Earth, to begin with? Was it seeded from space? Did it grow up around hydrothermal vents? Is it the product of random chance, or are certain evolutionary pathways favored over others?
If we hope to look for life in the cosmos, we need to know more about these basic questions. What is true for biology is also true for technology. At present, we are confined to looking for technosignatures we would recognize, which means ones we use ourselves or are within the realm of known physics. But if there are more advanced civilizations out there, couldn’t they be using technologies we haven’t conceived of yet?
After all, the existence of physics beyond the Standard Model — which advanced intelligence may have discovered a long time ago — could allow for all kinds of technologies we can’t conceive of yet. So really, our only option is to keep looking for what we would recognize as life and activity, and hope that the definition of “life as we know it” will gradually expand.
